Reed switch

ABSTRACT

The cooperating contacts of a reed switch are made of rhodium oxide. Such contacts are prepared by plating rhodium on the reeds of reed switches and then oxidizing the surface layers of the plated rhodium contacts by heating them in an oxygen containing atmosphere at a temperature of from 380* to 580* C for 3 to 15 minutes.

Toshiki et a1.

[ REED SWITCH 3,606,498 9 1971 Ohno 200 144 B x .7 MA V 7 19 S 2001 6 C[75] Inventors: Yokokawa Toshiki; Chihiro 5/ 72 temmetz 6 Kawakita, bothof Tokyo, Japan Primary Examiner-Robert K. Schaefer [73] Assignee: OKlElectric Industry Co., Ltd., Assistant Examiner wmiam J Smith TokyoJapan Attorney, Agent, or Firm chittick Thompson & 22 Filed: Jan. 26,1973 Pfund [21] Appl. No.: 327,098

[57] ABSTRACT U.S- Cl. 4 The ooperating contacts of a eed witch are madeof Clrhodi m xide Su h onta ts are prepared Fleld of Search J, rhodiumon the eeds of reed witches and then oxi- 200/166 1443 dizing thesurface layers of the plated rhodium contacts by heating them in anoxygen containing atl References Cited mosphere at a temperature of from380 to 580 C for UNITED STATES PATENTS 3 to 15 minutes. I i 3,222,489 121965 Ch k' 200 166 C 4 l m 2 Claims, 5 Drawing Figures fir I 51 May28,1974 PATENTEDIM 28 m4 SHEET 1 0f 3 F/GQ/ F/G.Z

START I NUMBER OF OPERATION F G 3 STAR] NUMBER OF UPERNTIRN BACKGROUNDOF THE INVENTION This invention relates to a reed switch having contactswhose contact resistance does not increase after a number of operationsand to a method of preparing such contacts.

A conventional reed switch comprises a sealed glass envelope 1, a pairof reeds 2 usually made of Fe-Ni Permalloy, and contacts 3 on the innerends of reeds 2 said contacts being usually formed by plating a nobelmetal.

Although rhodium has generally been used as the nobel metal because itis suitable for use as the contact material for reed switches, has adisadvantage in that the contact resistance between contacts of rhodiumincreases abnormally after a relatively small number of operations.Although a number of reasons for this may be considered, surfaceadsorption and catalytic action inherent to rhodium are considered to bethe major reasons. When contacts of rhodium whose surfaces have adsorbedvarious organic substances or other impurities are opened and closedmany times polymers are formed by the energy of collision of thecontacts any by the catalytic action of rhodium and such polymers causean increase in the contact resistance.

Accordingly, it is desirable to develop a novel method of surfacetreatment of the contact which can eliminate the catalytic action ofrhodium thereby providing reliable contacts free from any abnormalincrease in the contact resistance.

SUMMARY OF THE INVENTION It is an object of this invention to provide animproved reed switch provided with rhodium contacts which do notincrease their contact resistance during use.

Another object of this invention is to provide a novel method of surfacetreatment of the rhodium contacts of reed switches.

According to this invention, these and other objects can be accomplishedby oxidizing the rhodium contacts of a reed switch so as to formextremely thin films of rhodium oxide. To this end, the contacts platedwith thin films 'of rhodium are heated to a temperature of 480i 100C for3 to minutes in an oxygen containing atmosphere. The resulting thinfilms of rhodium oxide greatly reduce the adsorption of impurities andcatalytic action of metallic rhodium.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of theinvention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. I is a diagrammatic representation of a conventional reed switch;

F IG. 2 is a graph showing the variation in the contact resistance of aprior art reed switch having rhodium contacts;

FIG. 3 is a graph showing the variation in the contact resistance of therhodium contacts prepared in accordance with this invention;

FIG. 4 is a diagrammatic representation of a test device used todemonstrate the advantage of this invention; and

FIG. 5 is a graph showing the relationship between the deterioration ofthe contact resistance and baking temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT As above described, according tothis invention, contacts of reed switches are prepared by platingrhodium on the reeds of reed switches and then oxidizing the platedrhodium in an oxygen containing atmosphere under conditions describedabove. To compare the variation in the contact resistance of the priorart rhodium contacts and rhodium contacts having thin layers of rhodiumoxide which are prepared by the method of this invention, the followingexperiment was made. More particularly, as shown in FIG. 4, a dishshaped container 9 containing leads 7 for reed switches having contactsprepared by the method of this invention and another container 9containing reeds having prior art rhodium plated contacts were placed ona perforated shelf 6 of a sealed vessel 4. A quantity of benzol 5 wasplaced in the bottom of vessel 4 which was exhausted by means of avacuum pump (not shown) connected to an exhaust pipe 8. After 24 hours,the reeds 7 and 7 were taken out from the vessel 4 and were'respectively used to fabricate reed switches and life tests werecarried out for these reed switches. As shown in FIG. 2, the contactresistance of conventional contacts increased greatly after a relativelysmall number of operations, whereas the contact resistance of thecontacts treated in accordance with this invention varied only a littleafter many operations (that is 10) as shown in FIG. 3.

When inspected with a microscope,' the prior art contacts were observedto have formed polymers because they have adsorbed benzol which wasconverted into polymers by the energy of the collision of contacts andthe catalytic action of rhodium. On the other hand no polymer was notedon the contacts of reeds of this invention.

, FIG; 5 shows a preferred range of oxidation temperature, wherein theordinate shows the percentage of rejected contacts due to excessiveincrease in the contact resistance after 10 operations (which is atypical number of cycles for the life test) and the abscissa representsthe baking or oxidation temperature of plated rhodium contacts. As canbe noted from FIG. 5 with a baking temperature of from about 380C toabout 580C, the percentage of the rejected contacts is approximatelyzero but increases rapidly at temperatures below 380C and above 580C.This is because at temperatures below 380C the reaction is notsufficient whereas at temperatures above 580C the surface layer ofrhodium undergoes recrystallization due to heat or the diffusion of theplated metal occurs.

What is claimed is:

1. In a reed switch comprising a sealed envelope, a pair of reeds sealedto the wall of said envelope and a pair of cooperating contacts, theimprovement wherein said contacts comprise thin surface layers ofrhodium oxide operative to prevent surface contamination and withoutsubstantial increase in contact resistance.

2. The reed switch according to claim 1 wherein each of said contactscomprises a plated layer of rhodium and a thin layer of rhodium oxidethereon.

* it i 1|

2. The reed switch according to claim 1 wherein each of said contactscomprises a plated layer of rhodium and a thin layer of rhodium oxidethereon.